Five workers who handled the envelope were quarantined in the court as all other staff were evacuated just after 9am for fear of being contaminated by the mystery substance.

The workers were eventually taken to the Royal Surrey County Hospital decontamination unit four hours later after specialist police officers in biological suits ushered them from court. The five court workers were given yesterday (Thursday) off work.

Police sealed off the entire building for the rest of the day – but many evacuated workers were left frustrated as they could not access their vehicles in the court car park until early afternoon.

Prisoners who were set to appear before magistrates were put back into custody.

It was not known, as the Surrey Advertiser went to press, what the substance was or if the envelope carried a letter, but police said there was “no evidence” there was a risk to public health. Supt Kevin Deanus of Surrey Police said: “There is no indication of a terrorist threat here.

“The forensic examination of the substance will last several days, but we now know it is not harmful to the public.

“Public safety comes first so we had to ask everybody to leave the building and deal with those who came in contact with the substance. Any criminal investigation comes second.”

At one point in the morning four fire appliances, two ambulances and scores of police cars were at the scene as bewildered workers and passers-by gathered outside the court.

Phillip Baugh, security officer at the court, said: “We were obviously concerned about our colleagues. I arrived at 9am and the alarm had already gone off.

“I helped with the evacuation to make sure everybody else left the building – except for those who came in contact with the substance.”

No-one was allowed to re-enter the court building that day. Guildford Magistrates Court re-opened for business again yesterday (Thursday) morning – with a massive backlog of cases to deal with.

Shirley Gray, officer manager, said: “We are back to normal today but the room where the post is opened remains cordoned off.

“We have contingency plans in place for incidents like this but when it happens it is still shocking for everybody concerned. But the day was handled exceptionally well by everybody. We now have to deal with the backlog of work left.”